Problem: Before the previous stop there were 29 people riding on a train. 20 people ended up getting off at the stop. How many people are riding the train now?
Solution: The number riding the train now is the difference between the number who were riding and the number who got off at the previous stop. The difference is $29 - 20$ people on the train. $29 - 20 = 9$ people are on the train.